46 MICRO-ORGANISIMS AND DISEASE [CHAP. 



the flasks for about one-fourth to one-third of their bulk. 

 All these test-tubes and flasks with their cotton-wool plugs, 

 before receiving the material, should be thoroughly sterilised 

 by heating. As I mentioned in the previous chapter, this 

 ought to be well borne in mind, for starting with a sterile 

 nourishing fluid — i.e. one that has been kept in the stock 

 flask for several days to several weeks in the incubator at a 

 temperature of from 32° to 38° C. and that has remained 

 perfectly clear and limpid — and working with thoroughly 

 sterilised test-tubes and cotton-wool plugs — very little care 

 is required to obtain sterile material ready for inoculation. 

 To start with a stock of nourishing material, however well 

 sterilised, and to decant it into test-tubes with cotton-wool 

 plugs not absolutely sterile must lead to failure. I have 

 seen this happen over and over again, and all the material 

 decanted became consequently contaminated and thereby 

 useless for inoculations. The test-tubes, glass dishes, and 

 flasks must be well cleaned with strong acid, then well 

 washed with water, then dried, placed in the hot air-chamber, 

 and kept there exposed for several hours to a temperature 

 of from 130' to 150° C., or they may be thoroughly heated 

 in all parts over the open flame of a gas-burner. The test- 

 tubes and flasks are plugged by means of sterile forceps with 

 the cottoh-wool which is just faintly brown, and then replaced 

 in the air-chamber and again heated up to a temperature of 

 130° to 150° C. To decant sterile stock fluid into these 

 test-tubes and flasks I proceed thus : A clean beaker with 

 spout, covered with a clean glass plate, is placed inverted on 

 a net on a tripod over the flame of a Bunsen burner, and 

 thoroughly heated for half an hour or so ; then it is allowed 

 to cool, and when cool the plug of the stock flask is lifted 

 with forceps, and some of the sterile fluid quickly poured 

 from the flask into the beaker. The plug is replaced in the 



